In its most conventional form, a fabricated sheet metal "base" or "plain-Jane" wheel for a motor vehicle is comprised of a one-piece, dual flange, drop-center rim onto which a tire is mounted, and a flanged disc (also variously termed a "spider", "web", "dish", "body" or "center") that supports the rim at the disc flange and provides means for attachment to a spindle hub, brake drum or other like associated part of the vehicle. It is essential that the rim and disc, in their assembled relationship, insure perfect roundness of the rim and accurate axial alignment of the rim with respect to the disc, deviations in the respective directions being termed "radial" and "axial run-out." Vehicle manufacturers establish extremely rigid specifications in the tolerances for these dimensions. When such wheels are manufactured, the rim is normally made as a separate component from the disc and finish formed by profile rolling and final expansion, and the disc is formed in a progressive die stamping operation performed in a transfer press. These two elements are then press-fit assembled together and the disc fixed at its outer peripheral flange to the inner periphery of the rim by welding, riveting or some other like method to form the complete wheel assembly. In an ideal method for attaining the desired wheel roundness, the rim would be perfectly formed in the desired shape before assembly to the disc, and likewise accurate dimensional control would be achieved in making the finished disc. However, in practice, the conventional mass production manufacturing methods and equipment have not achieved such perfect roundness and desired accuracy in these separate wheel parts.
Typically when such wheels have been run with test overloads to induce failure, fatigue cracks have usually occurred in the center of the disc where it is attached to its supporting axle. When the wheels have been fabricated utilizing welding, failures have also occurred in the welds which have attached the rim to the disc. The failures in these areas of the wheel have been so apparent to the industry that an axiom has developed in the automotive industry that you do not machine or diminish the thickness of the disc to improve its accuracy, since it is the "weakest link in the chain".
On the other hand, it is well recognized that wheels are critical to the safety of an automotive vehicle. What makes the problem of wheel design even more difficult is that the analysis of stresses in the metal of the wheel is complicated by many factors which seemingly defy accurate appraisal, including tire unbalances, radial and lateral eccentricities of the rims which in turn support the tires, sufficiently loose tolerances in both starting material dimensions and composition and in manufacturing tooling and fixtures that are necessary for economical manufacture, and stresses that are created by the manufacturing processes used in forming the wheels. In the light of all these variables, and the inability to accurately calculate or predict the stresses involved even with modern finite element analysis computer modeling, the problem of how best to form a wheel in a commercially feasible manufacturing process, and at a cost which the average consumer can afford, has continued since the start of the automotive industry.
In order to improve the accuracy and strength of the finished wheel assembly, it is known to produce disc wheels of sheet steel from two parts by pressing a wheel disc made oversize in respect of the inside diameter of the well base of the rim and by subsequently welding the two parts to one another. Rim accuracy has been improved by various calibrating operations as by permanent deformation expanding or upsetting of the rim, to make it ready for fitting to the disc.
A multitude of U.S. Patents have issued for forming wheel rims. For example, the U.S. Pat. Nos. 2,586,029; 2,649,886; 2,826,161; 3,298,218; 3,564,898; 4,809,529; and 5,010,759 all disclose devices for forming wheel rims.
Furthermore, it is also known to subject completed disc and rim wheel assemblies, after the welding of the rim and wheel disc, to a subsequent truing operation by bending, to permanent set expanding or upsetting and, if appropriate, to additional machining, in order to improve the concentric and planar running of the wheel made from such cold-worked sheet metal parts. This is generally carried out in such a way that, by means of permanent deformation plastic cold working of the wheel mounting center region of the wheel disc and/or of the two rim tire bead seats, with mutual influencing of these two axial and radial reference planes, the geometrical variations are permanently reduced.
For example Gregg U.S. Pat. No. 3,530,717 discloses a machine for rounding a wheel having a disc attached to the wheel rim. The Gregg device has rounding dies to engage and shrink set the rim periphery, which is first formed oversize and permanently assembled to an oversize disc, for bringing the rim into axial and radial alignment. Additionally, when the rim is held in its trued position by the rounding dies, a machining or forming operation is performed upon a locating and/or mounting opening(s) in the disc so that the rim will be true with respect to its axis of rotation as defined by such opening(s). Additional prior art patents which have approached the problem of making such fabricated wheels with the requisite extremely accurate wheel configurations required by the automotive OEM customers, by reworking of the disc and/or rim after permanent assembly thereof, are the U.S. Pat. Nos. to Gollwitzer 3,580,043; Waterbury 3,581,550; Gregg 3,688,373; Roper 3,729,795; Phillip 3,855,837; Main et al 4,143,499; Trevarrow 4,304,034; and Zimmerman 4,378,623.
It can be appreciated that when radially loading an oversized rim to take a permanent set, as set forth in such prior art patents, the tendency will be for the disc and even the rim itself to spring back. Thus, such prior art shrink truing methods must provide for allowance to reduce the spring back. Nevertheless it is difficult if not impossible to completely eliminate the effect of spring back, and hence the disc center opening(s) may still not be located precisely centrally of the rim, and the desired mounting arrangement of the wheel also will not be achieved in the event that the disc center is not exactly parallel with the plane of the rim bead seats.
Another and commercially successful approach to improving wheel uniformity in such fabricated sheet metal wheel assemblies, but still requiring after-assembly processing, has been the non-deformation pierce-after techniques disclosed in the Daudi et al U.S. Pat. Nos. 4,279,287 and 4,354,407, assigned to Motor Wheel Corporation, assignee of record herein.
One well known source of the long-standing problem of inaccuracies resulting in the assembled wheel and disc, leading to such prior art efforts to resolve the problem by after-assembly rework and/or disc hole forming operations, has been the difficulty inherent in the preferred processes for economically forming the wheel disc from sheet metal blanks. In order to satisfy the need for mass production at high production rates, the wheel discs have been formed by cold working in progressive die stamping and draw tooling provided in multiple-stage, high speed transfer press production equipment. Many forming stages as well as subsequent manufacturing operations are often required to transform the flat circular sheet metal starting blank into the various configurations, contours and openings involved in providing the three primary zones of the disc, namely, the central bolt circle wheel mounting portion of the disc, the double reversely curved "window" or "beauty-section" of the disc which extends radially outwardly of the wheel from the center mounting portion, and the inboard curved outer flange portion which forms the disc-to-rim mounting and attachment zone of the disc. The need to progressively shape the disc into the basically different contours and functions of these three primary zones of the disc configuration in such a multiple-stage stamping operation poses complex die design and press transfer problems. For example, as many as eight or more stages may be required in the disc forming transfer press or specifically in separate press operations, e.g., (1) draw, (2) reduce, (3) form center and face, (4) trim O.D., (5) form edge, (6) pierce vent and medallion holes, (7) pierce bolt and center hole and (8) coin vent backside. See for example SAE Paper SP-897 entitled "Autobody Stamping Applications and Analysis" published February, 1992, pages 41-49, and in particular pages 47 and 48 thereof, as well as Swan U.S. Pat. No. 4,280,426 and Metals Handbook, 8th Edition, Vol. 4, pp. 182(FIG. 48), published by the American Society for Metals (1969).
It is to be understood that in the vehicle wheel art and as used herein, the aforementioned term "inboard", as well as its antonym "outboard", are used as directional and relative location adjectives or adverbs analogous to their meanings in nautical and aircraft usage. Thus, "inboard" as used in describing wheel structure means located, facing or extending toward the inside of the vehicle body and/or chassis (or the vehicle longitudinal center line or axis) on which the wheel is mounted in use, or in a position closer or closest to the vehicle longitudinal center axis relative to other structure or components of the wheel. "Outboard" is used in the opposite sense. The "outboard" side of the vehicle wheel is also variously referred to in the trade as the "curb" side, "beauty" side or "street" side. Also, the terms "front" and "rear" sides of the wheel are also used in the trade synonomously with the "outboard" and "inboard" sides of the wheel.
Yet another problem inherent in the progressive stamping of the wheel disc construction resides in the constraints imposed on the selection and design of the disc blank. The initial stock thickness must be as uniform as possible throughout the blank when starting from sheet metal material, but stock thickness variations inevitably occur. Material choice is also limited, i.e., mild carbon or HSLA steel in order to meet both manufacturing formability requirements and the strength and flexure characteristics capable of satisfying the severe fatigue load specifications of current automotive passenger vehicle wheels. Since the cyclical stress levels imposed on the wheel during use vary significantly as between the various portions of the wheel disc, the parameters of the disc blank as well as the ultimate cross-sectional configuration and contours of the wheel disc must be selected and designed to accommodate the fatigue life requirements of the most highly stressed areas. This can result in "material overdesign" with respect to the fatigue life requirements of the lower stressed areas of the disc, and create difficulties in the deep drawing of material of such strength and thickness into the desired disc flange shape.
As a result of these factors, variations in flange thickness and deviations from true cylindrical form (i.e., "waviness") may and often do occur in the disc flange. Attempts to solve this problem by constraining the flange after the "form flange" stage during subsequent disc forming operations in the transfer press pose additional problems of increased tooling cost and part transfer problems (e.g., part "sticking").
Another factor contributing to disc non-uniformity arises in producing one very common form of a "plain-Jane" wheel disc, namely a "scalloped" disc. Such a disc has a peripheral flange interrupted by "scallops" or "chain slots" so as to form "spokes" in the disc, usually four in number at equally spaced intervals around the disc flange. This disc configuration offers advantages in terms of reducing weight and requiring only four short disc-rim attachment fillet welds instead of a 360.degree. fillet weld at the disc flange inboard edge. Also, an octagonal starting blank design can be utilized which permits pattern nesting and resultant material cost savings, as well as "automatic" scallop formation. However, this type of interrupted or scalloped disc flange, as compared to the uninterrupted 360.degree. flange type disc also in common use, further aggravates the problems of distortion, thickeness non-uniformity and other dimensional inaccuracies in the uncontrolled disc flange as the disc is progressively finish formed in the transfer press.
When such scalloped or full flange discs are assembled to the rim by press fitting, they can and often do transfer the disc flange distortions to the rim, resulting in assembled wheel inaccuracies despite using accurately made rims. Hence the need hitherto for the aforementioned after-assembly processing techniques and equipment to correct wheel non-uniformity parameters, but which, however, also add significantly to wheel manufacturing costs. Moreover, despite such after-assembly correction processing, certain other manufacturing problems remain uncorrected. Disc flange distortion and thickness non-uniformities can cause disc-to-rim welding problems as well as paint bleed out and plating problems during final wheel coating operations due to poor matching fit of the disc in the rim. Although machining a true surface on the outer periphery of the disc flange might theoretically overcome these problems with some added cost, wheel fatigue life would seriously suffer due to weakening of the disc, and other distortions in the disc and/or rim can be induced during press-fitting due to flange thickness variations uncorrectable and/or introduced by such machining.